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Strings in Java are immutable. Basically this means that, once you create a string object, you won't be able to modify/change the content of a string. As a result, if you perform any manipulation on a string object which "appears to" change the content of the string, Java creates a new string object, and performs the manipulation on the newly created one.
Based on this, your code above appears to create five string objects - two are created by the declaration, two are created by calls to substring, and the last one is created after you concatenate the two pieces.
Immutability however leads to another interesting consequence. JVM internally maintains something like a string pool for creating string literals. For saving up memory, JVM will try to use string objects from this pool. Whenever you create a new string literal, JVM will loop into the pool to see if any existing strings can be used. If there is, JVM will simply use it and return it.
So, technically, before Java 7, JVM will create only one string object for your whole code. Even your substring calls won't create new string objects in the pool, it will use the existing "Hello World" one, but in this case it will only use characters from position 0 to 3 for your first call to substring, for example. Starting from Java 7, substring will not share the characters, but will create a new one. So, total object count will be 4 - the last one will be created with the concatenation of the two substrings.
Edit To answer your question in the comment, take a look at Java Language Specification -
In the Java programming language, unlike C, an array of char is not a String, and neither a String nor an array of char is terminated by '\u0000' (the NUL character).
A String object is immutable, that is, its contents never change, while an array of char has mutable elements.
The method toCharArray in class String returns an array of characters containing the same character sequence as a String. The class StringBuffer implements useful methods on mutable arrays of characters.
So, no, char arrays are not immutable in Java, they are mutable.
Literal a is created newly and kept in the pool.Literal b refer the a, it will not create new one instead.
The line 3 will create 3 new String since substring creates a new string and concatenate creates new Strings every time.
String substring(int beginIndex,int endIndex)
Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.
Hey guys, I've been working on this Java homework for my class and this whole section about string manipulation is confusing me.
The homework asks to add statements that use the substring method to get the first half and second half of phrase.
I'm assuming that means using the substring method, but I always get errors trying to compile what I put in. So here's the code. I'm not exactly sure how to use the substring method in the way they ask, and my book doesn't really go over it well. Should I be creating another variable or just use what I have?